In the context of a vehicle having a heat engine, the function of an alternator is conventionally to supply power to the wiring network of the vehicle and to recharge the battery of the vehicle when the engine is running. However, limitations are placed on the value of the voltage delivered by the alternator, and by the wiring system itself and the battery of the vehicle. In this regard, the output voltage of the alternator must generally vary as a function of temperature, for example according to a law similar to that illustrated in the curve shown in FIG. 1 of the accompanying drawings. FIG. 1 shows the ideal charging profile as a function of temperature, for a lead-acid battery.
In the example illustrated in FIG. 1, the optimum charge of the battery makes it necessary for the charging voltage to vary in a linear manner as a function of temperature over a straight line D having a negative slope (for example -33 mV/.degree. C.). It will be noted that the charging current undergoes severe variation once the charging voltage departs from this optimum line. It is therefore important that the charging voltage should follow this optimum line extremely closely.
In addition, the voltage delivered at the output of the alternator is limited to an upper value which is generally on the order of 15 to 16 volts, being 15.2 volts in the example shown in FIG. 1. This limitation is imposed in particular by the lighting equipment where this equipment makes use of filament bulbs. The alternator output voltage is further limited to a lower value, in the region of 13 volts, by the electromotive force of the battery when the latter is discharged.
Generally, in order to obtain regulation of this type, regulators are used which act on the excitation of the alternator as a function, firstly, of the temperature measured by a temperature sensor which is incorporated in the regulator of the alternator, and secondly, of the output voltage of the alternator or the charging voltage of the battery.
This way of achieving regulation is inexpensive, but from a technical point of view is not at all precise. In this regard, the alternator and the battery are separate components, situated in different places within the vehicle, and the variation in temperature of the battery is very slow, while that of the alternator (and therefore also of the regulator) is very rapid.
In order to be able to regulate the charging voltage of the battery correctly, it is desirable to know precisely the temperature of the battery. However, this is not the case with conventional regulators, which make use of the temperature measured directly on the alternator. In addition, regulators which make use of measurement of battery temperature, are expensive at the present time.